PTL 1 discloses a determination device configured to detect a rotation reference position of a rotating body according to rotation signals of the rotating body detected by a sensor. The sensor outputs the rotation signals every time the sensor faces multiple projections disposed on an outer periphery of the rotating body at regular intervals. A missing tooth part in which the projections are missing is disposed in the rotating body. The missing tooth part indicates a rotation reference position of the rotating body.
The determination device receives the rotation signal from the sensor at any time, and calculates a differential value of a ratio of time intervals of the rotation signals, that is, a time ratio of the rotation signals to compare the differential value with a threshold, thereby detecting the missing tooth part of the rotating body. In this way, the determination device detects the rotation reference position of the rotating body.
However, in the conventional art, the determination device employs a method of determining the missing tooth part with the use of the time ratio of the rotation signals. For that reason, timing at which the determination device receives the rotation signals from the sensor changes due to an instantaneous change in a rotation speed of the rotating body with the results that the differential value of the time ratio of the rotation signals is likely to exceed the threshold. Therefore, the determination device erroneously detects the missing tooth part, resulting in a risk that a sufficient detection precision of the rotation reference position of the rotating body is not obtained.